1. Field of the Invention
The application relates to a method utilized in a wireless communication system, and more particularly, to a method of preventing random access response collision in a wireless communication system.
2. Description of the Prior Art
Toward advanced high-speed wireless communication system, such as transmitting data in a higher peak data rate, LTE-Advanced system is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of LTE system. LTE-Advanced system targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), uplink multiple input multiple output (MIMO), etc.
For bandwidth extension, carrier aggregation is introduced to the LTE-Advanced system for extension to wider bandwidth, where two or more component carriers are aggregated, for supporting wider transmission bandwidths (for example up to 100 MHz) and for spectrum aggregation. According to carrier aggregation capability, multiple component carriers are aggregated into overall wider bandwidth, where the UE can establish multiple links corresponding to the multiple component carriers for simultaneously receiving and/or transmitting. In carrier aggregation, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information, and the security input. This cell is referred to as a primary cell (PCell). In the downlink, the component carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). In addition, cells other than the PCell are named secondary cell (SCell).
According to the specification of the LTE-Advanced system, a UE performs a random access procedure for synchronizing with a serving base station on uplink timing, to prevent, signals transmitted from the UE from colliding with those sent from other UEs under the coverage of the base station. For a non contention random access procedure, the network assigns a dedicated preamble to the UE for triggering the non contention random access procedure, and thereby the UE transmits the dedicated preamble to the network. After the network receives the dedicated preamble from the UE, the network transmits a random access response (RAR) including timing advance command for uplink synchronization to the UE. Note that, according to the agreement on LTE-Advance system specification, the UE may transmit the preamble on a SCell, but receives the RAR from the network on a PCell . In addition, the UE shall receive the RAR during a RAR window. If the UE does not receive the RAR during the RAR window, the random access procedure is considered failure. Moreover, the network uses a medium access control protocol data unit (MAC PDU) to transmit the RAR to the UE. The UE decodes the MAC PDU by a random access radio network temporary identifier (RA-RNTI) which is calculated by the UE and network according to a subframe where the preamble is transmitted.
However, the applicant notices problems associated to RAR collision. In detail, if two UEs transmit the same dedicated preambles respectively to the network for the non contention random access procedure at the same time, the two UEs derive the same RA-RNTI. As abovementioned, the RAR is transmitted only on a PCell. If the two UEs have the same PCell, both UEs decode the RAR at the PCell, and thereby causing RAR collision. One UE may not successfully perform uplink synchronization since timing advance command of the RAR is accurate only for one of the UEs.
Take an example based on the abovementioned concept. A UE1 is assigned with a dedicated preamble for transmission on a SCell A. In addition, a UE2 is assigned with the same dedicated preamble on a PCell A or a SCell B, where a PCell of the UE1 and the PCell A of UE2 are the same cell. The UE1 transmits the dedicated preamble to the network on the SCell A and meanwhile the UE2 transmits the dedicated preamble to the network on the PCell A or the SCell B. In this situation, the UE1 and UE2 derive the same RA-RNTI and thereby both decode the RAR transmitted by the network on the PCell A, which causes a RAR collision. Since the UE1 and UE2 do not know the RAR is for itself or not, the UE1 and UE2 may both apply the timing advance command and/or the uplink grant in the RAR. This may cause data transmission failed on one of the UE1 and UE2 or only one UE can be uplink synchronized with the network since the timing advance command and/or the uplink grant is accurate only for the UE1 or UE2.